1. Field of the Invention
This invention relates to apparatus for and a method of producing pellets from a water-containing feed material, and more particularly to such apparatus and method for dewatering and pelletizing feed materials containing an excess of water.
2. Description of the Prior Art
It is well known to form finely divided solid particles into pellets to facilitate handling and use of the products. Charcoal is formed into briquettes for use in backyard barbecues and metal ores are formed into pellets to be fed into smelting furnaces. Animal feeds are formed into pellets to minimize waste, and waste materials are pelletized to facilitate handling and disposal. Other pelletizing operations too numerous to mention are well know.
In many commercial pelletizing operations, the feed material is a product of a controlled manufacturing process and the composition is consistent so that pelletizing is easily controlled; in others, the feed composition varies so greatly that pelletizing must be carefully monitored and controlled. For example, it is well known that variations in the amount of water or liquid in a feed material will have a marked effect both on the ability to form the material into pellets and on the strength and integrity of the pellets formed. Commercial pelletizers may have feed blenders and mixers including means for the addition of water or other liquids as necessary to enable the formation of competent pellets; however, such devices do not include dewatering systems for the removal of excess moisture.
Substantial effort has been and continues to be expended to economically recover and use products such as coal fines, biomass and waste products as fuels, but these efforts have met with limited success. As a result, industry is compelled to spend substantial sums to dispose of such waste products, and the disposal procedures have not always been satisfactory. For example, the large volumes of coal fines produced in the washing of mined coal have an excessively high water content for economic recovery and use despite their potential fuel value. Such coal fines, which normally have a particle size of about 1 mm or less, are routinely collected in tailing ponds as waste. This practice is not only expensive but also harmful to the environment.
Similar problems plague other industries such as the paper and plastics industries where the substantial waste products are not easily disposed of. While furnaces have been designed to burn many such waste products as fuels, such furnaces are expensive both to design and to operate. Specialized materials handling equipment may be required and operation of the furnaces is erratic and difficult to control.
U.S. Pat. No. 5,743,924 discloses a pelletized fuel made from a mixture of finely divided coal particles, paper making sludge waste, waste plastics and waste paper, which products are mixed together and pelletized without the use of adhesives or binders. A commercial pelletized fuel plant constructed in accordance with this patent utilizes sludge from a paper making facility, in combination with finely divided coal and shredded plastic materials, and is utilized by the paper making facility as fuel. This commercial facility not only realizes substantial savings on the cost of fuel over the previously used coal, but also saves the cost of disposal of the waste sludge. Further, substantial reductions in SOx, NOx, and particulate emissions are realized. Since the present invention is particularly useful in pelletized materials of the type disclosed in Pat. No. 5,743,924, the disclosure of this patent is incorporated hereby by reference.
U.S. Pat. No. 5,658,357 discloses a process for producing coal logs or compacts from finely divided coal particles, utilizing laboratory procedures, but no commercial facility utilizing the process is believed to be in operation. The described process includes accurate control of the water content, accurate control of the zeta potential of the product, and the use of very high pressures over an extended time during which excess water may be expressed. The manner of expressing the water is not disclosed.
Slurry and sludge dewatering devices are also known in which excess water is extracted from a slurry or sludge pumped or forced through the devices. Examples of such dewatering devices may be found in U.S. Pat. Nos. 3,938,434; 4,798,194; and 5,173,196. These devices generally employ an elongated filtering chamber through which the solids/liquid mixture passes, with excess liquid being permitted to escape through filtered openings in the sidewalls of the chamber to produce an increased solids concentration or compact discharged from the end of the chamber. The problem of solid particles tending to plug the filter openings is discussed, for example, in U.S. Pat. No. 3,938,434.
While the pelletized fuel disclosed in the above-mentioned U.S. Pat. No. 5,743,924 represents a significant advance in the use of biomass and waste as a fuel, the processing costs and the moisture content of the pellets have been relatively high. These factors affect the strength and abrasion resistance of the pellets and limit the distance which the fuel can be economically transported. Further, the water content of the biomass and waste components must be monitored since excessive water content can result in the inability to form a satisfactory pellet. These factors make it difficult for such fuels to be price competitive with coal in the steam production and other markets. Similarly, excess liquid in the feed material can adversely affect other commercial pelletizing operations.
Accordingly, it is a primary object of the present invention to produce a pelletized material which may be produced at a lower cost and which can be processed into pellets having a relatively low moisture content and high strength.
Another object of the invention is to produce such a pelletized material which may incorporate a wide variety of biomass and/or waste material.
Another object of the invention is to provide a method of producing a pelletized product in which the material being pelletized is effectively dewatered during the pelletizing process.
Another object is to provide such a method in which the materials to be dewatered and pelletized are processed in a pelletizing die having water escape openings in the die walls to permit excess water to escape during the pressurization and pelletizing process.
Another object is to provide an improved pelletizing apparatus including a pelletizing die which permits the escape of excess moisture during the pelletizing process.
Another object is to provide an improved pelletizing die which may be installed in new or existing commercial pelletizing apparatus.
The foregoing objects and advantages of the invention are achieved in accordance with the present invention in which a pelletized product is produced from a finely divided pelletizable feed material, or a mixture of finely divided feed materials and water, by pressing the material through die openings in a commercial pelletizer such as a Kahl, California, or a Bliss pellet mill and permitting excess water to be expelled from the die during pelletizing. While the invention may be utilized in the processing of various raw materials, it has been found especially useful in producing a pelletized fuel from a finely divided material such as coal or a mixture of coal with a combustible biomass and/or waste material, and the invention will be described with specific reference to such materials, it being understood that the invention is not so limited.
The die of the improved pelletizer includes one or more water outlet openings preferably at a location, or locations, spaced upstream from the pellet discharge outlet of the die. As the material being processed is pressed through the die, pressure builds in the material progressively from the entrance to a point adjacent the outlet. This build-up of pressure causes excess water in the material to flow through the openings in the die wall. Continued movement of the material expels the dewatered and pelletized material from the die openings in the conventional manner. In one embodiment, a pressure enhancing means is provided for increasing the pressure in the material as it passes through the dies to enhance dewatering and produce a more compact pellet.
The dimensions of the water outlet openings in the die walls are determined by the nature of the material being processed as well as the material particle size. The water outlets generally may be substantially greater than the minimum particle size of the material being processed, and where the material contains an element of fibrous material such as paper pulp sludge or the like, the openings may be greater than for materials consisting essentially of solid particulates such as coal fines. The pressurization/dewatering process which enables the use of die wall openings greater than the particle size without expelling excessive particles from the openings cannot be determined for certain, but it is believed the particles tend to bridge the openings in a manner to permit the water under high pressure to be expelled without the particles entering and plugging the openings. The expelled water is collected and led from the pellet mill for discharge as waste or for further treatment.
In accordance with one embodiment of the invention, the die of a pellet mill such as a Kahl pelletizer may be made as a plurality of plates assembled together as by bolting, with the water outlets being provided by the space between adjacent plates, which space can be controlled as by the surface finish of mating plate surfaces, or the use of shims, spacers, or the like between the plates.
In accordance with another embodiment, a back pressure plate is supported in closely spaced relation to the outlet of the pellet mill, with the back pressure plate being driven for rotation about the pellet mill axis at a rate different from the rate of the pressure rolls of the mill. The back pressure plate has a substantially flat surface with spaced openings extending therethrough so that, in operation, the flat surface will underlie a portion only of the die openings while the plate openings will underlie the remaining die openings. By driving the pressure rolls and back pressure plate at different speeds, a portion of the die outlets will be blocked as the pressure rolls pass thereover on each revolution, thereby increasing the pressure in the blocked dies. Continued operation will result in these die openings being unobstructed on a subsequent pass, or passes, of the pressure roll to expel the dense pellets.